In order to perform health management for prophylaxis and treatment, it is frequently required to determine the presence of various components in the blood of the patient. For example, the level of glucose or cholesterol in the blood is important for effective prophylaxis of various diseases such as diabetes, hypoglycemia, liver trouble, thyroid trouble, atherosclerosis, etc.
Hitherto, these components in the blood have been determined in the serum or plasma after removing the red blood corpuscles from the blood. However, in order to avoid this troublesome operation and the increase in cost for the equipment required to separate the red blood corpuscles from the other components of the blood, it is preferred that the components being analyzed in the whole blood can be determined without separating the red blood corpuscles. Also, if undiluted whole blood can be used for the analysis, samples can be more simply and quickly obtained and assayed. This is particularly important for the health management in a home or a practitioner's office, etc., wherein the analysis is required to be as simple as possible.
A dry chemical analysis is known (i.e., clinical analysis using an analytical element in a substantially dry state, such as a test piece or a multilayer analytical element, containing therein anlytical reagent(s)). Dry chemical analysis is excellent as compared to wet chemical analysis (i.e., a method using analytical reagent(s) dissolved in a solution) in terms of easiness of use, economy, and quickness of analysis. However, in the case of analyzing whole blood using conventional dry chemical analysis, it is required to first remove the blood corpuscles (red blood corpuscles and white blood corpuscles) and other unnecessary high molecular components from the sample or to separate these unnecessary components from the sample within the analytical element by some means.
In such an analysis, the necessity of isolating the serum or plasma from the blood corpuscles in the whole blood must be avoided because the operation of removing the blood corpuscles from the serum or plasma using a filter element requires a very long time and much trouble. In addition, part of the serum or plasma is inevitably lost in the filter element when such an assay component passes through the filter element, which possibly makes the analysis incorrect.
Some conventional dry analytical elements require removing the blood corpuscle components by, after permeating the serum or plasma into the analytical element, wiping off the blood corpuscle components. The analysis of whole blood is possible by using a multilayer analytical element having a porous layer which catches the blood corpuscle components occupying a large portion of the whole blood whereby the serum or plasma can pass through into a reagent layer containing a reagent causing a detectable change in the presence of a specific component.
U.S. Pat. No. 4,042,335 describes whole blood analysis using a multilayer analytical element. The multilayer analytical element described therein is composed of a support having formed thereon, in succession, a recording layer, a light blocking layer, and a reagent layer. The reagent layer can also function as a porous spreading layer and the light blocking layer can also function as a barrier an impermeable layer capable of removing the whole blood corpuscles so as to prevent the corpuscles from reaching the recording layer, whereby disturbances caused by hemoglobin can be avoided (see, FIG. 1 of the aforesaid U.S. Patent).
The aforesaid analytical element can measure assay materials in whole blood, however, has various problems. First, a detectable material which can quickly diffuse through the corpuscle impermeable layer into the recording layer and has a sufficiently high extinction must be used. However, only a few of detectable material (dyes, coloring agents, etc.) can simultaneously satisfy the aforesaid requirements.
Second, in the case of separating corpuscle components within the analytical element, the porosity or the size of pores of the surface with which the blood sample is brought into contact must be sufficiently large to completely absorb the sample. However, if the pore structure is too rough, the analysis element is mechanically unstable, i.e., is liable to be broken.